Mesoporous Melamine-Formaldehyde Resins as Efficient Heterogeneous Catalysts for Continuous Synthesis of Cyclic Carbonates from Epoxides and Gaseous CO₂

Herein, we report the application of inexpensive mesoporous melamine-formaldehyde resins (MMFR and MMFR250) obtained by a novel template-free and organosolvent-free hydrothermal method as efficient heterogeneous catalysts for direct synthesis of cyclic carbonates from CO 2 and epoxides (epichlorohydrin, butylene oxide, and styrene oxide). The catalytic activities of the melamine resins were attributed to the abundant Lewis basic N-sites capable of activating CO 2 molecules. Based on CO 2 -temperature programmed desorption, the concentrations of surface basic sites for MMFR and MMFR250 were estimated to be 172 and 56 μmol/g, while the activation energies of CO 2 desorption (strength of basic sites) were calculated to be 92.1 and 64.5 kJ/mol. We also observed considerable differences in the catalytic activities and stabilities of polymeric catalysts in batch and in continuous-flow mode due to the existence of a synergism between adsorption of CO 2 and cyclic carbonates (poison). Our experiments also revealed the important role of catalyst surface chemistry and CO 2 partial pressure upon catalyst poisoning. Nevertheless, owing to their unique properties (large specific surface area, large mesoporous, and CO 2 basicity), melamine resins presented excellent activities (turnover frequency 207–2147 h –1 ) and selectivities (>99%) for carbonation of epoxides with CO 2 (20 bar initial CO 2 or CO 2:epoxide mole ratio ∼1.5) under solvent-free and co-catalyst-free conditions at 100–120 °C. Most importantly, these low-cost polymeric catalysts were reusable and demonstrated exceptional stability in a flow reactor (tested up to 13 days of time on stream, weight hourly space velocity 0.26–1.91 h –1 ) for continuous cyclic carbonate production from gaseous CO 2 with different epoxides (conversion 76–100% and selectivity >99%) under industrially relevant conditions (120 °C, 13 bar, solvent-free/co-catalyst-free) confirming their superiority over the previously reported catalytic materials.